1. Field of the Invention
The present invention relates generally to apparatus and methods for generating electrical pulses, and more particularly for recharging a string of avalanche transistors within a pulse generator designed to produce very high voltage, current, and power pulses of a very brief duration.
2. Discussion of Background Art
Pulse generators are versatile devices which are widely used throughout the electronics industry and are incorporated into many different electronic products. Each pulse generator however needs to be tailored to the needs of its intended application and thus the circuitry within any particular pulse generator may vary greatly.
In the field of optics, pulse generators are commonly used to drive Pockels cells. Pockels cells are widely known in the art as electro-optic light modulators which modify light wave polarization in response to voltages. In response to very high voltage short duration pulses, Pockels cells can act like switches. Such switches have significant application to laser based systems and can be used in laser machining tools. Laser machining tools use short duration laser pulses to machine surfaces. The shorter the laser pulse the less heat generated.
FIG. 1 is a circuit diagram of a prior art apparatus 100 for recharging avalanche transistors 102 and 104 useful for driving Pockels cells. The avalanche transistors are biased near an avalanche operating point by a high-voltage power supply 106 and a single charge resistor 108 connected to a first output capacitor 110 through a Zener diode 112 and directly to a second output capacitor 114. In response to an input trigger signal 116, the circuit 100 generates and transmits a high-voltage, short-duration output pulse to an output load 118, such as a Pockels cell. After generating a pulse, capacitors (C1) and (C2) are fully discharged. The capacitors 110 and 114 are then recharged by current flowing from the high-voltage power supply 106. The power supply 106 charges each of the capacitors 110 and 114 through the charge resistor 108 and the Zener diode 112. At first Zener diode 112 is off and only (C2) 114 is charging. When the voltage across (C2) 114 reaches the Zener voltage of diode 112, diode 112 turns on and both (C1) 110 and (C2) 114 are then charging. Once, both capacitors 110 and 114 are charged, the pulse generator circuit 100 is ready to generate a next pulse at the output 118.
A paper entitled Design of Reliable High Voltage Avalanche Transistor Pulsers, by E. S. Fulkerson, R. Booth and presented at the 21st International Power Modulator Symposium at Costa Mesa, Calif. in June of 1994 contains additional details on the operation of the prior art apparatus and is incorporated herein by reference. One manufacturer of pulse generators using strings of avalanche transistors, such as shown in FIG. 1, is "Kentec," a British Company, which produce 5 kV pulses with 100 picosecond widths.
The circuit in FIG. 1 operates at about a 5 kHz frequency and is useful for some applications. This slow maximum pulse rate is limited by the ability of the circuit 100 to recharge the output capacitors 110 and 114. Industry, however, is demanding more effective laser machining tools capable of operating faster than 5 kHz.
In response to the concerns discussed above, what is needed is an apparatus and method for recharging a string of avalanche transistors within a pulse generator that operates at higher frequencies.